The printing plate material for CTP, which is inexpensive, can be easily handled, and has a printing ability comparable with that of a PS plate, is required accompanied with the digitization of printing data.
Recently, a thermal processless printing plate material which can be applied to a printing press employing a direct imaging (DI) process without development by a special developing agent or a versatile thermal processless printing plate material which can be treated in the same manner as in PS plates has been required.
As the thermal processless printing plate material for DI, there is a Thermo Lite produced by Agfa Co., Ltd. However, this plate material requires development on press, and when the plate is mounted on a printing press, and printing is carried out in the same printing sequence as in a conventional PS plate, good initial printability cannot be obtained. Further, this plate has problem in that stain occurs during printing due to some combination of dampening water and printing ink, and is not so high in versatility. Furthermore, this plate does not have an exposure visualization property, since plate inspection after thermal laser exposure is not considered.
Properties required for a versatile processless printing plate are good initial printability, in which printing can start under the same printing conditions as a conventional PS plate (without any special development on a printing press); broad versatility, in which a conventional dampening water or printing ink used in the PS plate can be also used; and an image visualization property after imagewise exposure.
It is expected in the CTP system that procedure of plate inspection will be not carried out in future. However, the plate inspection is still necessary in the present processing procedures. Therefore, in the thermal processless printing plate material, image visualization after image recording is one of the important performances.
The thermal processless type printing plate material is divided into an ablation type printing plate material, and a development-on-press type heat fusible image formation printing plate material from the viewpoint of the image formation mechanism. The ablation type printing plate material has problems that it is low in sensitivity due to its image recording mechanism, and it is necessary to provide, in an exposure device, a sucking device for preventing a part of the image formation layer from scattering during laser exposure of the printing plate material.
The development-on-press type heat fusible image formation printing plate material is more advantageous than the ablation type printing plate material, in that it is high in sensitivity, and it does not scatter a part of the image formation layer during laser exposure of the printing plate material.
As the development-on-press type heat fusible image formation printing plate material, a printing plate material disclosed in JP-2938397 is cited which comprises a hydrophilic layer or a grained aluminum plate and provided thereon, an image formation layer containing thermoplastic particles and a water soluble binder. The Thermo Lite described above produced by Agfa Co., Ltd. is this type of a processless printing plate material.
In the development-on-press type heat fusible image formation printing plate material, developability on press or printing performance such as initial printability or anti-stain property is greatly influenced by kinds of materials contained in the image formation layer. A dye used as a light-to-heat conversion material or a dye precursor or discoloring agent for providing exposure visualization has a great influence on the printing performance. This is probably because the dye, or the dye precursor or discoloring agent is strongly adsorbed on the grained surface of the aluminum plate, and is difficult to remove with a dampening water and/or printing ink.
A planographic printing plate material is proposed which employs a light-to-heat conversion material and improves initial printability and anti-stain property. For example, a planographic printing plate material is proposed which comprises a substrate and provided thereon, an image formation layer containing an infrared absorbing dye, and an outermost layer in that order, wherein on imagewise exposure, the outermost layer forms hydrophilic portions and a hydrophobic portions resulting in image formation, and the optical density of the image formation layer varies (see, for example, Japanese Patent O.P.I. Publication No. 11-240270). Further, a planographic printing plate material is proposed which comprises a hydrophilic substrate and provided thereon, an image formation layer containing thermoplastic particles comprised of a homopolymer or copolymer of styrene and a hydrophilic binder having a carboxyl group, wherein the image formation layer or its adjacent layer comprised of a heat sensitive image formation composition containing an anionic infrared cyanine dye (see, for example, Japanese Patent O.P.I. Publication No. 11-265062).
However, they neither improve initial printability nor an anti-stain property. Further, neither of the references refers to conception that improves initial printability and an anti-stain property by optimizing the surface configuration of an aluminum support as a support of a printing plate material.
As is described above, the conventional processless printing plate materials do not have good initial printability, good anti-stain property, and exposure visualization.